Polhippo has a good point carry sick bags.
I also get them out for the student and move the microphone out of the way. Nothing worse than a chunky microphone sock. I've never had anyone need to wash out the aircraft afterwards.
Other things 'students' have done
Landed with the feet on the brakes though over concentration on the task of landing.
One guy who had a lapsed licence came for a trial lesson. He wanted to fly but not use the checklist (thats my job apparently) . He then randomly flicked switches when he felt the need in the cockpit. We had words.
An experienced tailwheel pilot who wanted a chippy reval.I thought I was along for the ride until he almost groundlooped it. I sat a bit closer to the controls after that.
Another experienced pilot who tried to round out 20 feet too high. -Admittedly it was windy. You expect students to do it, again I sat closer to the controls.
And its not just in the air.
One experienced pilot who tried to start the aircraft while I was checking the oil. Luckily I heard the fuel pump running and managed to get out of the way. I adjusted my walkround routine after that.
All a massive learning experience for me as much as them.